1. Field of the invention:
The present invention relates to an improved active-type auto-focusing mechanism which can measure the distance between an object and film surface without any failure in distance measurement due to any partial reflection at the object or incorrect light beam projection from the light source.
2. Description of the Prior Art:
Recently, auto-focusing mechanisms have been developed for use in, for example, cameras, etc. to automatically measure the distance between an object or objects and a film surface. Various types of such auto-focusing mechanism have been proposed. The present invention utilizes the principle of trigonometric distance measurement in the auto-focusing mechanism. The trigonometric distance-measurement will be first described below.
The trigonometric distance-measurement is schematically shown in FIG. 1. As seen, a light beam is projected from a light source 6 to an object 4 through a light-projection lens 8. The light beam reflected at the object 4 is made incident upon a photo detector 5 through a photo-detection lens 9. Since the base-line length B is predetermined, the distance between the light-projection lens 8 and object 4 can be measured by determining the deviation y of the incident beam spot from the optical axis .alpha. in the photo-detection plane or photo detector 5. That is a simple geometry can be used to determine the distance L from the photo-detection lens 8 to the object 4 based on the similitude between the triangles AOC and ECD. This can be expressed as follows: EQU L=(B/y).multidot.CE (a)
where
CE: Distance between photo-detection lens and plane PA1 B=AC: Base length
These values CE, B and AC are predetermined. Therefore, if the deviation y is known in the expression (a), the distance L can be determined.
Next, a camera incorporating an auto-focusing mechanism which can automatically measure a distance between an object and film surface under the above-mentioned principle, will be described. As shown in FIGS. 1 and 2, such camera has a finder 105 on opposite sides of which a light projector 1 and photo detector 2 are provided, respectively. A beam of, for example, infrared rays or the like is projected from the light projector 1 toward the object 4 and the infrared beam reflected at the object 4 is incident upon the photo detector 2. For detection of the above-mentioned incident infrared beam, the photo detector 2 is of an active-type auto-focusing mechanism using a position-sensitive device which utilizes the lateral photo effect on the semiconductor surface (will be referred to as "PSD type" hereinafter), position-sensitive element on which a predetermined pattern of light shields is provided (will be referred to as "pattern type" hereinafter), or the like.
In case of the pattern type system, the photo detector 2 is composed, as shown, for example, in FIG. 3, of a reference photo-detection element 10 disposed in the image plane of the photo-detection lens 9' and intended for canceling the reflection factor of the object, and a position-sensitive photo-detection element 11 disposed in the image plane of the photo-detection lens 9 and having on the photo-detection surface thereof an aluminum-made wedge-like light shield which detects the position of the reflected light beam from the object. By determining the ratio in light amount between the position-sensitive and reference photo-detection elements 11 and 10, the position of the incident beam spot F can be detected irrespectively of the reflection factor of the object.
Since the base-line length is the data indispensable for the distance measurement and depends upon the position of the incident beam spot F, it is important to accurately detect the position of the incident beam spot F.
Generally, the light beam has a circular section and is projected toward the object so that the incident beam upon the photo detector has also a circular section. As shown in FIG. 3, the Y-directional distance y.sub.2 between the central position M(end point) in the photo detection plane of the beam spot F and an arbitrary point(base point) on the optical axis is determined as the correct base length.
In practice, however, even if the light beam of a circular section is projected toward the object, it may possibly be incident upon the photo-detection plane of the reference and position-sensitive photo-detection elements 10 and 11, respectively, taking a distorted form losing the circularity when reflected at the object 4.
In case the section of the incident light beam is not circular as in the above, a processor provided calculates the center of mass of the incident beam spot based on its form in the photo-detection plane. The base-line length in the auto-focusing mechanism is so set as to be calculated taking the calculated center of mass as an end point. Thus, when the incident light beam F has a circular section, the center of mass may possibly deviate from the central position, resulting in an erroneous distance measurement.
In such conditions, if the diameter W of the projected light beam is large as shown in, for example, FIG. 4, the incident light beam has no circular section. The light beam reflected at the object has a noncircular section when incident upon the photo detector (will be referred to as "partial reflection" hereinafter), so that no accurate distance measurement is possible. So it is desirable to reduce the diameter of W of the projected light beam as much as possible.
However, if the diameter of the projected light beam is made small, the measuring range is limited correspondingly. For example, in case two objects standing side by side are photographed while viewing the middle between them, the light beam will pass through between them as shown with .epsilon. in FIG. 4 and go further to the background at the back of the objects (this will be referred to as "incorrect reflection" hereinafter). In this case, it is not possible to get any in-focus photo-graph of the objects.